Process for manufacturing prepreg

ABSTRACT

A process for manufacturing a prepreg exhibiting excellent formability and producing laminated boards and multiple layer circuit boards exhibiting high thickness precision is disclosed. The process of manufacturing the prepreg comprises (a) a step of impregnating a glass fiber substrate with a solvent, (b) a step of impregnating the solvent-impregnated glass fiber substrate with epoxy resin, (c) a step of heating the epoxy resin-impregnated glass fiber substrate, (d) a step of further impregnating the epoxy resin-impregnated glass fiber substrate, in which the epoxy resin has been cured, with the epoxy resin, and (e) a step of heating the lastly obtained epoxy resin-impregnated glass fiber substrate, wherein the epoxy resin reaction rate in the inner layer is 85% or more and the epoxy resin reaction rate in the outer layer is 60% or less.

TECHNICAL FIELD

[0001] The present invention relates to a process for manufacturing ahigh quality prepreg having high thickness precision, exhibitingexcellent formability, and suffering from only minimal fall out of resinpowder. Particularly, a prepreg obtained by a process of the presentinvention can be suitably used for preparing substrates for cellularphones, personal computers, Rambus memories, PDAs, and the like forwhich high-frequency characteristics are required.

BACKGROUND ART

[0002] Recently, as higher frequencies than before are used forlaminated sheets for circuit boards and multiple-layer circuit boards,the requirements for the properties of the materials used for suchboards have become more stringent. Materials for circuit boards haveconventionally been studied, particularly with respect to circuit signaldelays. Resins capable of controlling impedance through a reduction inthe dielectric constant of circuit boards and the increase in thethickness precision of the substrates after fabrication have beendeveloped. The thickness precision of a substrate correlates with signaldelays, and the square root of the dielectric constant correlates withthe signal delays. Therefore, increasing the thickness precision of asubstrate is an important subject. It has been difficult to provideconventional prepregs with high substrate thickness precision due toflow out of the resins impregnated and cured in glass fiber during pressfabrication. Only poor substrate thickness precision can be achieved ifthe newest type press facility is not used. In addition, resin powderreadily falls from prepregs when the prepregs are cut or bent duringhandling. Such a resin powder adheres to copper foils and causes circuitdefects.

[0003] Methods for improving the substrate thickness precision have beendisclosed in Japanese Patent Applications Laid-open No. 123875/1978, No.142576/1979, No. 168438/1988, and No. 119836/1992. Japanese PatentApplications Laid-open No. 123875/1978, No. 142576/1979, and No.119836/1992 disclose prepregs containing a completely cured resin layerand a partly cured resin layer. The dimensional stability of laminatedboards has been improved by using these methods. However, these methodshave a problem of peel-off of the resin layers at the interface of acompletely cured resin layer and a partly cured resin layer. JapanesePatent Application Laid-open No. 168438/1988 discloses a prepregcomposed of resin layers having a different reaction rate. This methodprovides only insufficient improvement in the dimensional stability whenapplied to the fabrication of laminated boards. In addition, the methodmay cause migration of voids in the inner cloth, resulting in impairedlong-term reliability.

[0004] A method of improving the problem of resin powder production fromprepregs when bending the substrates has been disclosed in JapanesePatent Publication 334/1994. The Japanese Patent Publication 334/1994proposes a method of melting the parts from which resin powder mayreadily fall out or the parts to which the resin powder has becomeattached. Although this method can prevent production of resin powderfrom prepregs, the method has problems such as denaturing of epoxy resindue to melting, requirement of investment for equipment, and an increasein the number of process steps.

[0005] Therefore, an object of the present invention is to provide aprocess for manufacturing a prepreg having high thickness precision,free from production of resin powder by bending and the like, free fromvoids in the inner layers, free from flow out, and exhibiting excellentformability.

DISCLOSURE OF THE INVENTION

[0006] In view of this situation, the inventors of the present inventionhave conducted extensive studies and have found that a prepreg havinghigh thickness precision, free from production of resin powder bybending and the like, free from voids in the inner layers, free fromflow out, and exhibiting excellent formability can be obtained by aprocess comprising a step of impregnating a glass fiber substrate with asolvent, a step of impregnating the solvent-impregnated glass fibersubstrate with epoxy resin, a step of heating the epoxyresin-impregnated glass fiber substrate under specific conditions, astep of further impregnating the epoxy resin-impregnated glass fibersubstrate, in which the epoxy resin has been cured, with the epoxyresin, and a step of heating the last-obtained epoxy resin-impregnatedglass fiber substrate under specific conditions, wherein the epoxy resinreaction rate in the inner layer is 85% or more and the epoxy resinreaction rate in the outer layer is 60% or less. These findings have ledto the completion of the present invention.

[0007] Specifically, the present invention provides a process ofmanufacturing a prepreg comprising (a) a step of impregnating a glassfiber substrate with a solvent, (b) a step of impregnating thesolvent-impregnated glass fiber substrate with epoxy resin, (c) a stepof heating the epoxy resin-impregnated glass fiber substrate, (d) a stepof further impregnating the epoxy resin-impregnated glass fibersubstrate, in which the epoxy resin has been cured, with the epoxyresin, and (e) a step of heating the lastly obtained epoxyresin-impregnated glass fiber substrate, wherein the epoxy resinreaction rate in the inner layer is 85% or more and the epoxy resinreaction rate in the outer layer is 60% or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a flow diagram showing process steps for manufacturingthe prepreg of an embodiment of the present invention.

DETAILED DESCRIPTION

[0009] The process for manufacturing the prepreg of the presentinvention will now be explained referring to FIG. 1. In the FIG. 1, theprepreg manufacturing device 12 comprises an unwinder 1 which unwinds aglass fiber substrate, a first accumulator 11 a, a solvent impregnationapparatus 3, a solvent impregnation region 4, a first resin varnishimpregnation apparatus 5, a first drier 6, a second resin varnishimpregnation apparatus 7, a resin amount control equipment 8, a seconddrier 9, a second accumulator 11 b, and a winder 10. These apparatusesare arranged in that order from the upstream to downstream.

[0010] The step (a) comprises unwinding glass fiber substrate 2 from theunwinder 1 and impregnating the glass fiber substrate 2 with a solventto decrease voids in the target prepreg. There are no specificlimitations to the glass fiber substrate 2, for example, known glassfiber substrate for prepreg can be used as the glass fiber substrate 2.A conventionally known thread opening treatment and a convergent agentremoving treatment may be preferably carried out to increase epoxy resinimpregnation in the inner layer, ensuring manufacture of voidlessprepregs (hereafter may be called “voidless”). The thread openingtreatment used in this specification means a treatment using a highpressure water jet or the like to loosen warp yarn and weft yarn formingthe glass fiber substrate and expand the width between them. Theconvergent agent removing treatment is a treatment to remove binders,glue materials, and the like conventionally used when a glass fibersubstrate is woven and to reduce the content of these materials to acertain level, e.g. to less than 0.1 wt %.

[0011] As a solvent used in a step (a), dimethyl formamide (DMF), methylcellosolve (MCS), methyl ethyl ketone (MEK), and the like can be given.Of these solvents, DMF is preferably used due to easy control in themanufacturing process, excellent impregnation in the glass fibersubstrate 2, and easy replacement with an epoxy resin in the later step.The time required for impregnating the glass fiber substrate 2 with thesolvent at this step is preferably 0.1 to 1 minute, and more preferably0.1 to 0.5 minute at room temperature. If the impregnation time is lessthan 0.1 minute, impregnation of solvent becomes insufficient. Animpregnation time longer than one minute requires a large andcomplicated facility for the solvent impregnation, giving rise toincreased production cost. Dip coating, kiss coating, and the like canbe given as a method for impregnating the glass fiber substrate with asolvent.

[0012] After the solvent impregnation according to the step (a) andbefore impregnation of the inner layer with the epoxy resin according tothe step (b), the substrate is preferably passed through an air streamfor 1 to 5 minutes at a temperature lower than the boiling point of thesolvent used in the solvent impregnation according to the step (a),preferably at least 50° C. lower than the boiling point of the solventused in the solvent impregnation according to the step (a). Thisprocedure helps the solvent to homogeneously spread all over the glassfiber and ensures a voidless inner layer. After this substrate is passedthrough the air stream, an excess amount of solvent is removed by acollecting rod or the like so that the amount immediately before thestep (b) becomes 16 to 25 parts by weight per 100 parts by weight of theglass fiber substrate. If there is an excess amount of solvent, theexcessive solvent is carried over to the resin varnish used in the step(b), making it necessary to remove the excessive solvent in the dryingoperation in the step (c). This accompanies undesirable problems such aseasy production of voids and the like.

[0013] In the step (b), the solvent-impregnated glass fiber substrate 2obtained in the step (a) is passed through the first resin varnishimpregnation apparatus 5 to impregnate the glass fiber substrate 2 withan epoxy resin. Examples of epoxy resins used in the step (b) include,but are not limited to, bisphenol epoxy resins and novolak epoxy resins.The epoxy resin is used in the form of a varnish formed by dissolvingthe epoxy resin in a solvent which preferably has good solubility withthe resin. A poor solvent may also be used to the extent there is noadverse effect. Additives such as a curing agent, curing catalyst,filler, surfactant, silane coupling agent, and the like can beoptionally added. The time required for impregnating the glass fibersubstrate 2 with the epoxy resin is preferably 0.1 to 1 minute, and morepreferably 0.1 to 0.5 minute at room temperature. If the impregnationtime is less than 0.1 minute, impregnation of the resin in the widthdirection may not be homogeneous. An impregnation time longer than oneminute requires a large and complicated facility for the varnishimpregnation, giving rise to increased production cost.

[0014] The step (c) is a step of heating the glass fiber substrate 2impregnated with the epoxy resin in a first drier 6 to form an epoxyresin layer which has reacted to a certain extent. In this step, voidsremaining in the cloth are removed by replacing the solvent with thevarnish and by heating. This procedure increases the reaction rate ofthe epoxy resin in the inner layer to 85% or more and makes the innerlayer voidless, thereby improving the thickness precision of theresulting prepreg and preventing production of resin powder. The heatingconditions are preferably 140 to 200° C. for 1 to 5 minutes. If theheating temperature is lower than 140° C., it takes too long for thereaction rate to reach 85%, requiring large scale production facilities.If the temperature is higher than 200° C., the epoxy resin may bedenatured, giving rise to impaired properties of the resulting laminatedproducts, particularly in terms of heat-resistance and water-absorptionresistance. If the heating time is less than one minute, voids mayremain in the inner layer; if the heating time is longer than fiveminutes, large production facilities may be required. In view ofcompatibility of the productivity and voids removal, desirable heatingconditions are heating temperatures of 160 to 180° C. and heating timesof 1 to 4 minutes. The reaction rate of 85% or more of the epoxy resinin the inner layer may not necessarily be achieved in this step (c). Itis sufficient that this target is ultimately attained in the prepregafter the later-described step (e).

[0015] The weight ratio (R)/(G) of the epoxy resin (R) and the glassfiber substrate (G) in the inner layer is 0.43 or more, and preferablyfrom 0.53 to 2.40. Specifically, the amount of epoxy resin shouldpreferably be 43 parts by weight or more for 100 parts by weight of theglass fiber substrate. If the amount of epoxy resin is less than 43parts by weight, impregnation of the resin in the glass fiber substrateis incomplete, giving rise to a risk of leaving strand voids. If such aninner layer is dried and cured, and an outer layer is coated over theinner layer, voids in the cloth cannot be sufficiently removed. Thereliability of the formed laminated boards may be impaired.

[0016] In the step (d), the epoxy resin-impregnated glass fibersubstrate 2 is passed through the second resin varnish impregnationapparatus 7 for further impregnation of the epoxy resin, whereby anouter epoxy resin layer is formed on at least one side of the innerlayer. The further epoxy resin impregnation after the above-mentionedstep (c) according to the present invention ensures the configurationcomprising an inner layer in which the reaction has proceeded to acertain extent and an outer layer on at least one side of the innerlayer. There are no specific limitations to the epoxy resin used for theouter layer. Bisphenol epoxy resins, novolak epoxy resins, and the likecan be given as examples. The time required for impregnating the epoxyresin-impregnated glass fiber substrate with an epoxy resin in this step(d) is preferably 0.1 to 1 minute at room temperature. If theimpregnation time is less than 0.1 minute, the resin may not beimpregnated in the width direction. An impregnation time longer than oneminute requires a large and complicated facility for varnishimpregnation so that production is hindered. The time in which thevarnish is impregnated is preferably from 0.1 to 0.5 minute in view ofproductivity. The amount of resin can be controlled by the resin amountcontrol equipment 8, such as a squeeze roll, a comma knife, or the like.Specifically, the resin amount can be controlled by the gap of the resinamount control equipment 8 and passage position of the glass fibersubstrate 2. For example, a prepreg with an equivalent outer layer resinamount on both sides can be obtained by passing the glass fibersubstrate 2 through the middle of the gap of the resin amount controlequipment 8.

[0017] The step (e) consists of heating the glass fiber substrate 2impregnated with the epoxy resin in a second drier 9 to form an outerepoxy resin layer. The reaction rate of epoxy resin in the inner layerand the outer layer is also adjusted in this step. The heatingconditions are preferably 140 to 200° C. for 1 to 5 minutes. If thetemperature is lower than 140° C., the productivity is poor; if higherthan 200° C., the curing reaction may proceed unnecessarily and theepoxy resin may be denatured, giving rise to impaired properties of theresulting laminated products, particularly in terms of heat-resistanceand water-absorption resistance. If the heating time is less than oneminute, the degree of curing in the width direction may become uneven,making it difficult to produce prepregs. Heating time in excess of fiveminutes requires large facilities. From the viewpoint of theproductivity, desirable heating conditions are a heating temperature of160 to 200° C. and a heating time of 1 to 4 minutes.

[0018] The prepreg of the present invention can be preferably used forcopper-clad laminated boards and multiple-layer print circuitboards(hereafter may be called “multiple-layer boards”). When theprepreg is used for insulating the circuit layers in multiple-layerprint circuit boards, the weight of the epoxy resin per unit area ineach outer layer should preferably be greater than the value Adetermined by the following formula and less than 1.5A.

A=(1-b/10²)×c/10⁴ ×d  (1)

[0019] wherein A is the weight of the resin per unit area of the outerlayer (g/cm²), b is the amount of residual copper (%) in a circuit layerfacing the outer layer of the prepreg, c indicates the thickness (μm) ofthe circuit copper foil, and d is the specific gravity (g/cm³) of theouter layer resin.

[0020] If the weight of epoxy resin per unit area in the outer layer isless than the value A, the amount of resin in the outer layer isinsufficient so that embedding of the circuits tends to be inadequate;if greater than 1.5A, on the other hand, the amount of resin in theouter layer is excessive, resulting in an increased flow-out amountduring a forming operation, which gives rise to impaired thicknessprecision. The effect on flow-out in different circuits having the samecopper residual rate differs according to the circuit patterns.Therefore, the amount of resin can be suitably determined in the aboverange. A preferable range for the weight of epoxy resin per unit area inthe outer layer is between 1.1A to 1.4A. The residual copper rate (%)indicates the percentage of the circuit' area formed in the innercircuit layer of a multiple-layer board.

[0021] When coating and impregnating the outer layer resin on both sidesof the inner layer, the amount of epoxy resin applied to the differentside of the inner layer may not be identical. For example, when copperfoil surfaces on which an outer layer is provided have different copperresidual rates, the thickness of the outer layer may be controlledaccording to the copper residual rate. specifically, the amount of resinapplied to the surface with a high copper residual rate may be smallerthan the amount of resin applied to the surface with a low copperresidual rate. Providing different amounts of resin to the differentsides of the outer layer ensures reduction of the flow-out amount. Whenjoining the prepreg of the present invention with a copper foil, theamount of resin in the outer layer can be determined according to theamount required for filling fine microscopic pits on the roughenedcopper foil surface. To produce the two outer layers each having adifferent amount of epoxy resin, the glass fiber substrate may be passedthrough the one side of a resin amount-controlling device 8 whichremoves a larger amount of resin from that side than from the otherside.

[0022] The reaction rate of the epoxy resin in the inner layer and theouter layer in the prepreg of the present invention is respectivelybrought to 85% or more and 60% or less by performing the above steps (a)to (e) in that order. A prepreg with a different reaction rate in theinner layer and the outer layer can be formed by suitably selecting theheating conditions of the step (c) and the step (e). Impregnation of theglass fiber substrate with a solvent in a proper and sufficientquantity, followed by replacement of the solvent with an epoxy resin,and formation of the outer and inner layers by epoxy resins with adifferent reaction rate according to the present invention ensureimprovement of thickness precision, prevent production of resin powder,and provide a voidless prepreg. If the reaction rate of the epoxy resinin the inner layer is less than 85%, an increased amount of resin flowsout during heat forming, resulting in impaired thickness precision. Onthe other hand, if the reaction rate of the epoxy resin in the outerlayer is more than 60%, adhesion with the other layers may becomeinsufficient when used for multi-layer boards, such as copper-cladlaminates and multiple-layer printed circuit boards, resulting ininadequate formability such as insufficient embedding of resins incircuits. Because the inner layer epoxy resin in the present inventionhas been cured to a more advanced degree, the prepreg exhibits onlylower fluidity and less adhesion if the inner layer alone is provided.However, the provision of the outer layer epoxy resin having thereaction rate of 60% or less can sufficiently achieve the object ofembedding of resin in the inner layer circuits and adhesion with otherlayers in the case of multiple-layer boards.

[0023] More preferably, the reaction rate of the inner layer epoxy resinis 90 to 95% and that of the outer layer epoxy resin is 0 to 20%; withparticularly preferable reaction rates for the inner layer and outerlayer being respectively 90 to 95% and 0 to 20%. If the reaction ratesare within the above preferable range, not only can the thicknessprecision be increased and resin powder production be prevented, butalso flow out of the resin can be prevented and formability can beimproved. In addition, if the reaction rate of the outer layer epoxyresin is 0 to 20%, embedding of the resin in the inner layer circuits inthe case of multiple-layer boards can be improved, saving the amount ofresin required for embedding or the amount of flowing resin. Thisresults in increased thickness precision. In a prepreg in which thereaction rate of the inner layer epoxy resin is not as high as requiredin the present invention or in a single layer prepreg with the reactionrate of 20% or less, flow out of resin increases and the substratethickness precision is impaired.

[0024] The reaction rate in the present invention can be determined bydifferential scanning calorimetry (DSC). Specifically, the reaction ratecan be determined by applying the exothermic peak areas due to the DSCreaction of both the unreacted resin and the resin in each layer to thefollowing formula (2). The measurement may be carried out in a nitrogenatmosphere at a temperature rise of 10° C./minute. $\begin{matrix}{{{Reaction}\quad {rate}\quad (\%)} = {\left\{ {1 - \frac{\left( {{reaction}\quad {peak}\quad {area}\quad {of}\quad {resin}} \right)}{\left( {{reaction}\quad {peak}\quad {area}\quad {of}\quad {unreacted}\quad {resin}} \right)}} \right\} \times 100}} & (2)\end{matrix}$

[0025] Although the reaction rate can be controlled by various methodssuch as adjustment of the heating temperature, heating time, irradiationof lights and electron beams, and the like, control by means of theheating temperature and heating time is easy and brings about a goodresult.

[0026] The prepreg obtained is sent to the second accumulator 11 b, andcontinuously wound by the winder 10 or cut to a prescribed length by acutter, not shown in the FIGURE. Specifically, the prepreg prepared inthis embodiment can be cut into an appropriate length, laminated with ametal foil, an inner layer circuit board, and the like, and fabricatedinto circuit boards or multiple-layer circuit boards by press-formingwith heating. Alternatively, the long prepreg may be wound as is andfabricated into circuit boards or multiple-layer circuit boards bycontinuously laminating with a metal foil such as a copper foil,aluminium foil, or nickel foil, an inner layer circuit board, and thelike.

EXAMPLES

[0027] The present invention will be described in more detail byexamples, which should not be construed as limiting the presentinvention.

Example 1

[0028] Using a prepreg manufacturing apparatus shown in FIG. 1, aprepreg consisting of an inner layer and an outer layer was prepared.The inner layer made from a commercial glass fiber substrate impregnatedwith an epoxy resin, both sides of which have been coated with an epoxyresin outer layer.

[0029] <Preparation of Epoxy Resin Varnish (I)>

[0030] 70 parts by weight of a bisphenol A epoxy resin with an epoxyequivalent of about 450 and 30 parts by weight of a phenol novolak epoxyresin with an epoxy equivalent of about 190 were dissolved in 100 partsby weight of methyl ethyl ketone. To this solution, a solution of 3parts by weight of dicyandiamide and 0.15 parts by weight of2-phenyl-4-methylimidazole dissolved in 20 parts by weight ofdimethylformamide was added. The mixture was stirred to obtain an epoxyresin varnish (I) for glass fabric.

[0031] <Preparation of Epoxy Resin Varnish (II)>

[0032] 35 parts by weight of a bisphenol A epoxy resin with an epoxyequivalent of about 450, 35 parts by weight of a bisphenol A epoxy resinwith an epoxy equivalent of about 2000, and 30 parts by weight of ano-cresol novolak epoxy resin with an epoxy equivalent of about 210 weredissolved in 100 parts by weight of methyl ethyl ketone. This solutionwas mixed with a solution of 3 parts by weight of dicyandiamide and 0.15parts by weight of 2-phenyl-4-methylimidazole in 20 parts by weight ofdimethylformamide to obtain an epoxy resin varnish (II).

[0033] <Preparation of Prepreg>

[0034] <Step (a)-step(c)>

[0035] The glass fabric reeled out from the unwinder 1 was passedthrough the solvent impregnation apparatus 3, where glass fabric wasimpregnated with DMF solvent (step (a)), and was passed through an airstream at 25° C. for 3.5 minutes. The solvent-impregnated-glass fabricwas then passed through the first resin varnish impregnation apparatus 5containing an epoxy resin varnish (I) to impregnate 100 parts by weightof the glass fabric with 64 parts by weight of the resin on the solidbasis (step (b)). The product from step (b) was dried for 3 minutes inthe first drier 6 at 170° C., to prepare an inner layer a consisting ofa glass fabric impregnated with the epoxy resin (step (c)).

[0036] <Step (d)-step (e)>

[0037] The epoxy-impregnated glass fabric obtained in the step (c) waspassed through the second resin varnish impregnation apparatus 7containing the epoxy resin varnish (I) to impregnate 100 parts by weightof the glass fabric with 110 parts by weight of the resin (including theweight of inner layer resin) on the solid basis (step (d)), followed bydrying for 1.5 minutes in a drier at 170° C. to form an outer layer b(step (e)). A prepreg consisting of an inner layer a and an outer layerb formed on both sides of the inner layer was obtained in this manner.

[0038] <Confirmation of Reaction Rate>

[0039] An inner layer, obtained by impregnating the glass fabric withthe epoxy resin varnish (I) and drying in a drier at 170° C. for 4.5minutes as mentioned above, was used as a sample for the inner layer a.The sample for the outer layer b was prepared by cutting the surface ofthe prepreg consisting of the inner layer a and the outer layer bobtained above. A heat generation peak in samples for each layer wasdetermined by DSC apparatus (manufactured by TA Instrument Co.). Theheat generation peak areas due to the curing reaction at about 160° C.for the resin before the reaction and the resin for each layer werecompared, and the reaction rate was calculated from the above formula(2). As a result, the reaction rates for the inner layer a and the outerlayer b were confirmed to be 88% and 59%, respectively.

Example 2

[0040] A prepreg consisting of an inner layer a and an outer layer bformed on both sides of the inner layer a was prepared in the samemanner as in Example 1, except that the inner layer b was formed byusing the epoxy resin varnish (II) instead of the epoxy resin varnish(I) in step (d). As a result, the reaction rates for the inner layer aand the outer layer b were confirmed to be 86% and 52%, respectively.

Comparative Example 1

[0041] A prepreg having only an inner layer a was prepared, omitting thesteps for forming an outer layer b. Specifically, in the steps (a) to(c), the solvent-impregnated glass fabric was passed through the firstresin varnish impregnation apparatus 5 containing the epoxy resinvarnish (I) to impregnate 100 parts by weight of glass fabric with 110parts by weight of the resin on a dry basis. The varnish-impregnatedglass fabric was dried in a drier at 170° C. for 1.5 minutes to obtain aprepreg. The reaction rate was 57%.

Comparative Example 2

[0042] A prepreg was prepared in the same manner as in ComparativeExample 1, except that the varnish-impregnated glass fabric was dried ina drier at 170° C. for 3 minutes instead of drying at 170° C. for 1.5minutes. The reaction rate was 77%.

Comparative Example 3

[0043] A prepreg consisting of an inner layer a with a reaction rate of53% and an outer layer b with a reaction rate of 70% was prepared in thesame manner as in Example 1, except for employing appropriately modifiedheating conditions in the step (c) and step (e).

Comparative Example 4

[0044] A prepreg was prepared in the same manner as in Example 1 exceptthat 100 parts by weight of glass fabric was impregnated with 30 partsby weight of the resin on a dry basis in the formation of the innerlayer a in the step (b), and 100 parts by weight of glass fabric wasimpregnated with 80 parts by weight of the resin on a dry basis in theformation of the outer layer b in the step (d). As a result, thereaction rates for the inner layer a and the outer layer b wereconfirmed to be 75% and 70%, respectively.

[0045] (Evaluation of Prepreg)

[0046] A double-sided copper-clad laminated board and a four-layercircuit board were prepared using the prepreg prepared in Examples 1 and2 and Comparative Examples 1 to 4. The properties of the boards wereevaluated. The results are shown in Table 1. In Table 1, fall-off ofresin powder is a result of the evaluation of the prepreg beforepreparing the double-sided copper-clad laminated board and thefour-layer circuit board.

[0047] <A. Preparation of Double-Sided Copper-Clad Laminated Board>

[0048] A copper foil with a thickness of 18 μm was layered on both sidesof the prepreg, press-formed with heating at a temperature 170° C. for60 minutes under a pressure of 40 kgf/cm², to obtain a double-sidedcopper-clad laminate with an insulating layer thickness of 0.1 mm.

[0049] <Evaluation of Double-Sided Copper-Clad Laminate>

[0050] The substrate thickness precision and formability were evaluatedon the insulating layer obtained by etching and removing the copper foilfrom the double-sided copper-clad laminate with a dimension of 500mm×500 mm. For the determination of substrate thickness precision, thesubstrate was divided into 36 squares and the thickness was measured ateach of the 36 squares. The average of the 36 measurements was taken asthe substrate thickness and the standard deviation of the 36measurements was taken as the substrate thickness precision. Theformability was evaluated by inspecting a 500 mm×500 mm board for thepresence or absence of voids in a given test circuit area and furtherinspecting the board for any abnormalities both by the naked eye and byusing an optical microscope.

[0051] 18 μm copper foil peel strength was evaluated according to JISC6481.

[0052] For the evaluation of solder heat resistance, only one side ofthe prepreg was etched and cut into a 50 mm×50 mm sheet, therebyobtaining three test specimens. Each test specimen was subjected to amoisture absorbing treatment in a pressure cooker at 121° C. under 2.0atm for two hours. The test specimens were then dipped in a solder bathat 260° C. for 120 seconds, after which they were inspected for thepresence of swelling and measles by the naked eye and opticalmicroscope.

[0053] For the evaluation of flow-out, a double-sided copper-cladlaminate was prepared and the flow-out length (the extruded area) wasmeasured.

[0054] <B. Preparation of Four-Layer Circuit Board>

[0055] An oxidation (blackening) treatment was performed on the surfaceof a copper foil (thickness: 35 μm) of a double-sided copper-cladlaminate with a thickness of 1 mm, as an inner layer circuit board. Onesheet of the prepreg was layered on each side of the copper-cladlaminate, over which a 18 μm copper foil was layered. The resultingboard was press-formed with heating at a temperature of 170° C. for 120minutes under a pressure of 40 kgf/cm² to obtain a four-layer circuitboard.

[0056] <Evaluation of the Four-Layer Circuit Board>

[0057] For the determination of the substrate thickness precision, a 500mm×500 mm substrate was divided into 36 squares and the thickness wasmeasured for each of the 36 squares. The average of the 36 measurementswas taken as the substrate thickness and the standard deviation of the36 measurements was taken as the substrate thickness precision.

[0058] The formability was evaluated by inspecting a 500 mm×500 mm boardfor the presence or absence of voids in a given test circuit area andfurther inspecting the board for any abnormalities both by the naked eyeand by using an optical microscope.

[0059] For the determination of inner layer peel strength, the peelstrength at the interface of the inner layer copper foil of thissubstrate, after the blackening treatment, and the prepreg was measured.

[0060] The solder heat resistance was measured in the same manner as inthe above-mentioned method of evaluation of the double-sided copper-cladlaminate.

[0061] For the evaluation of flow-out, a four-layer circuit board wasprepared and the flow-out length (the extruded area) was measured.

[0062] Fall-out of resin powder was evaluated by the following method.

[0063] First, a prepreg is cut to a size of 100 mm×100 mm using a handcutter taking as much care as possible to minimize resin powder fallingout from the cut end. Ten sheets of prepreg thus obtained were stackedand the weight was determined. The stack was then dropped ten times froma height of 100 mm to determine the weight of resin powder falling outby drop impact. The amount of fallen-out resin powder was determinedfrom the weight difference for the sheets of prepreg before dropping andafter dropping. Such a measurement was repeated five times to determinethe average. TABLE 1 Examples Comparative Examples 1 2 1 2 3 4Double-sided copper-clad laminated board Substrate thickness (avg.) mm0.153 0.154 0.152 0.155 0.154 0.152 Substrate thickness precision mm0.02 0.016 0.071 0.012 0.049 0.066 Formability (contact) No voids Novoids No voids Many voids Some voids Some voids Inner-layer peelstrength kN/m 1.43 1.50 1.41 0.52 1.42 1.43 Solder heat resistance n = 3◯◯◯ ◯◯◯ ◯◯◯ XXX ◯◯◯ ◯◯◯ Flow-out mm 0 0 7 0 0 2 Four-layer circuit boardSubstrate thickness (avg.) mm 1.287 1.294 1.268 1.298 1.295 1.282Substrate thickness precision mm 0.027 0.020 0.105 0.019 0.062 0.083Formability (contact) No voids No voids No voids Many voids Some voidsSome voids Inner-layer peel strength kN/m 0.85 0.98 0.83 0.31 0.85 0.86Solder heat resistance n = 3 ◯◯◯ ◯◯◯ ◯◯◯ XXX ◯◯◯ ◯◯◯ Flow-out mm 0 0 100 0 4 Prepreg Fallen-out resin powder g 0.007 0.008 0.024 0.003 0.0130.018

[0064] The prepregs obtained in Examples 1 and 2 exhibit high substratethickness precision and excellent formability. The product ofComparative Example 1, which is an example of a conventional prepreg,exhibited a substrate thickness precision worse than the prepregs of theExamples. The prepreg of Comparative Example 2 has a surface resin layerin which the reaction has proceeded excessively. The prepreg exhibitedgood thickness precision, but inferior formability. The prepreg ofComparative Example 3, in which the reaction rate in the inner layer ais lower than the reaction rate in the outer layer b, exhibitedsubstrate thickness precision worse than the prepregs of the Examples.The prepreg of Comparative Example 4, in which the reaction rate in theouter layer b is high and the amount of the resin is large, exhibitedsubstrate thickness precision worse than the prepregs of the Examples.

[0065] Industrial Applicability

[0066] The prepreg having high thickness precision, free from productionof resin powder by bending and the like, from voids in the cloth in theinner layers, and from flow out, and exhibiting excellent formabilitycan be manufactured securely and in a stable manner using themanufacturing process of the present invention.

1. A process for manufacturing a prepreg comprising (a) a step ofimpregnating a glass fiber substrate with a solvent, (b) a step ofimpregnating the solvent-impregnated glass fiber substrate with epoxyresin, (c) a step of heating the epoxy resin-impregnated glass fibersubstrate, (d) a step of further impregnating the epoxyresin-impregnated glass fiber substrate, in which the epoxy resin hasbeen cured, with the epoxy resin, and (e) a step of heating thelast-obtained epoxy resin-impregnated glass fiber substrate, wherein theepoxy resin reaction rate in the inner layer is 85% or more and theepoxy resin reaction rate in the outer layer is 60% or less.
 2. Theprocess for manufacturing a prepreg according to claim 1, wherein thereaction rate of the epoxy resin in the inner layer is 90 to 95% andthat of the outer layer is 0 to 20%.
 3. The process for manufacturing aprepreg according to claim 1, wherein the solvent impregnated in theglass fiber substrate in the step (a) is dimethyl formamide, methylcellosolve, or methyl ethyl ketone.
 4. The process for manufacturing aprepreg according to claim 1, wherein the time for which the solvent isimpregnated in the glass fiber substrate in the step (a) is 0.1 to 1minute at room temperature.
 5. The process for manufacturing a prepregaccording to claim 1, wherein, after the step (a) and before the step(b), the solvent-impregnated glass fiber substrate obtained in the step(a) is passed through an air stream for 1 to 5 minutes at a temperaturelower than the boiling point of the solvent used in the step (a).
 6. Theprocess for manufacturing a prepreg according to claim 5, wherein theamount of solvent in the solvent-impregnated glass fiber substrate afterpassing the substrate through an air stream and before the step (b) is16 to 25 parts by weight per 100 parts by weight of the glass fibersubstrate.
 7. The process for manufacturing a prepreg according to claim1, wherein the time for which the epoxy resin is impregnated in theglass fiber substrate in the step (b) is 0.1 to 1 minute at roomtemperature.
 8. The process for manufacturing a prepreg according toclaim 1, wherein the heating conditions in the step (c) are 140 to 200°C. for 1 to 5 minutes.
 9. The process for manufacturing a prepregaccording to claim 1, wherein the time for which the epoxy resin isimpregnated in the glass fiber substrate in the step (d) is 0.1 to 1minute at room temperature.
 10. The process for manufacturing a prepregaccording to claim 1, wherein the heating conditions under which theepoxy resin impregnated glass fiber substrate in the step (e) are 140 to200° C. for 1 to 5 minutes.